1. Field of the Invention
The invention relates to a method of measuring the movement of an input device and an object relative to each other along at least one measuring axis, the method comprising the steps of:
illuminating an object surface with a measuring laser beam for each measuring axis, and
converting a selected portion of the measuring beam radiation reflected by the object surface into an electric signal representative of the movement along said measuring axis.
The invention also relates to an input device provided with an optical module for carrying out the method, and to an apparatus comprising such an input device.
2. Description of the Related Art
Such a method and input device known from European Patent Application No. EP-A 0 942 285, corresponding to U.S. Pat. Nos. 6,246,482, 6,330,057, 6,424,407 and 6,452,683. The input device may be an optical mouse used in a computer configuration to move a cursor across the computer display or monitor, for example, to select a function of a displayed menu. Such an optical mouse is moved across a mouse pad by hand, like a conventional mechanical mouse. As described in EP-A 0 942 285, the input device may also be an xe2x80x9cinvertedxe2x80x9d optical mouse. The input device is then stationary and, for example, built in the keyboard of a desktop, notebook or palm computer and a human finger is moved over, for example, a transparent window in the housing of the input device. In the latter case, the input device may be small, because the optical module for measuring the finger movement can be made very small. In fact, the input device is reduced to the optical measuring module. This opens the way to new applications for the input device. For example, an input function can be built in a mobile phone for selecting functions on a menu and for accessing Internet pages, or in a remote control device for a TV set for the same purposes, or in a virtual pen.
EP-A 0 942 285 discloses several embodiments of the optical measuring module, in all of which homodyne or heterodyne detection is used. All embodiments comprise a diffraction grating arranged close to the module window. The grating reflects a portion of the illumination beam radiation, preferably radiation diffracted in one of the first orders, to a detector which also receives a portion of the radiation reflected and scattered by the object surface. The laser radiation diffracted in the first order by the grating is denoted a local oscillator beam, and the detector coherently detects the radiation from the object surface using this local oscillator beam. The interference of the local oscillator beam and the radiation reflected by the object surface reaching the detector gives rise to a beat signal from the detector, this beat signal being determined by the relative motion of the object surface parallel to this surface. The optical measuring module of EP-A 0 942 285 comprises, besides the grating, a collimator lens, a focusing lens and a pinhole diaphragm, preceding the detector, these elements having to be aligned very accurately. This complicates the manufacture and increases the cost of the module, which is intended to be a mass-produced consumer product.
It is an object of the invention to provide a method as described in the opening paragraph, which is based on another detection principle and allows the use of an optical configuration with fewer components, and is easier to manufacture. This method is characterized in that measuring beam radiation reflected back along the measuring beam and re-entering the laser cavity emitting the measuring beam, is selected, and in that changes in operation of the laser cavity, which are due to interference of the re-entering radiation and the optical wave in the laser cavity and are representative of the movement, are measured.
This new method of measuring the relative movement of an input device and an object, for example, a human finger or another object, uses the so-called self-mixing effect in a diode laser. This is the phenomenon that radiation emitted by a diode laser and re-entering the cavity of the diode laser induces a variation in the gain of the laser and thus in the radiation emitted by the laser. The object and the input device are moved relative to each other such that the direction of movement has a component in the direction of the laser beam. Upon movement of the object and the input device, the radiation scattered by the object gets a frequency different from the frequency of the radiation illuminating the object, because of the Doppler effect. Part of the scattered light is focused on the diode laser by the same lens that focuses the illumination beam on the object. Because some of the scattered radiation enters the laser cavity through the laser mirror, interference of light takes place in the laser. This gives rise to fundamental changes in the properties of the laser and the emitted radiation. Parameters which change due to the self-coupling effect, are the power, the frequency and the line width of the laser radiation and the laser threshold gain. The result of the interference in the laser cavity is a fluctuation of the values of these parameters with a frequency that is equal to the difference of the two radiation frequencies. This difference is proportional to the velocity of the object. Thus the velocity of the object and, by integrating over time, the displacement of the object can be determined by measuring the value of one of these parameters. This method can be carried out with only a few and simple components and does not require accurate alignment of these components.
The use of the self-mixing effect for measuring velocities of objects, or, in general, solids and fluids, is known per se. By way of example, reference is made to the article: xe2x80x9cSmall laser Doppler velocimeter based on the self-mixing effect in a diode laserxe2x80x9d in Applied Optics, Vol. 27, No. 2, Jan. 15, 1988, pages 379-385, and the article. xe2x80x9cLaser Doppler velocimeter based on the self-mixing effect in a fiber-coupled semiconductor laser: theoryxe2x80x9d in Applied Optics, Vol. 31, No.8, Jun. 20, 1992, pages 3401-3408. However, up to now, use of the self-mixing effect in an input device as defined above has not been suggested. This new application is based on the recognition that a measuring module using the self-coupling effect can be made so small and cheap that it can be installed easily and without much additional cost in existing devices and apparatus.
In order to detect the direction of movement, i.e., to detect whether the object moves forward or backward along the measuring axis, the method may be characterized in that the shape of the signal representing the variation in operation of the laser cavity is determined. This signal is an asymmetric signal and the asymmetry for a forward movement is different from the asymmetry for a backward movement.
Under circumstances where it is difficult to determine the asymmetry of the self-mixing signal, preferably another method is used. This method is characterized in that the direction of movement along said at least one measuring axis is determined by supplying the laser cavity with a periodically varying electric current and comparing first and second measuring signals with each other, these first and second measuring signals being generated during alternating first half-periods and second half-periods, respectively.
The wavelength of the radiation emitted by a diode laser increases, and thus the frequency of this radiation decreases, with increasing temperature, thus with increasing current through the diode laser. A periodically varying current through the diode laser in combination with radiation from the object re-entering the laser cavity results in a number of radiation pulses per half-period and thus in a corresponding number of pulses in the measured signal. If there is no relative movement of the input device and the object, the number of signal pulses is the same in each half-period. If the device and the object move relative to each other, the number of pulses in one half-period is larger or smaller than the number of pulses in the next half-period, depending on the direction of movement. By comparing the signal measured during one half-period with the signal measured during the next half-period, not only the velocity of the movement, but also the direction of the movement can be determined.
This method may be further characterized in that the first and second measuring signals are subtracted from each other.
The method of measuring the movement is preferably further characterized in that it is used to perform a click action by a single movement of the object and the input device relative to each other along an axis which is substantially perpendicular to the object surface.
The single movement also results in a Doppler shift of radiation of the measuring beam scattered and reflected by the object surface towards the laser cavity so that it can be determined whether the single movement has been performed by measuring the change of a relevant parameter of the laser cavity. After a cursor of, for example, a computer has been placed, on a desired function of a displayed menu chart under the control of the X- and Y-movement measuring systems of the input device, this function can be activated by the single movement in the Z-direction.
The method of measuring is preferably further characterized in that it is used to determine both a scroll action and a click action by movement of the object and the input device relative to each other in a first direction parallel to the object surface and in a second direction substantially perpendicular to the object surface.
A scroll action is understood to mean an up/down, or down/up, movement of a cursor across a menu. Such an action can be realized by moving a finger in a given direction over the input device. With this method, measurements along a first measurement axis, parallel to the object surface, and a second measurement axis, substantially parallel to the object surface, may be carried out. The first measurement furnishes information about the scroll-action and the second measurement furnishes information about the click action. Alternatively the two measurement axes may be at opposite angles relative to a normal to the object surface. Then the signals of the two measurement axes both comprise information about the scroll action and the click action. The specific scroll action information as well as the specific click action information can be isolated by appropriate combining the signals of the two measuring actions.
The changes in the operation of the laser cavity can be determined in several ways.
A first embodiment of the measuring method is characterized in that the impedance of the diode laser cavity is measured.
The impedance of the laser diode is one of the parameters which change due to the interference effect, and is a function of the relative movement of the input device and the object. This impedance can be determined by measuring the voltage across the diode laser and dividing the measured voltage value by the known value of the electric current sent through the diode laser.
A preferred embodiment of the method is characterized in that the intensity of the laser radiation is measured.
Measuring the intensity of the laser radiation is the simplest way of determining the changes in the laser cavity, because this can be done with a simple photo diode.
The invention also relates to an input device provided with an optical module for carrying out the method, this module comprising at least one laser, having a laser cavity, for generating a measuring beam, optical means for converging the measuring beam in a plane near the object, and converting means for converting a measuring beam radiation reflected by the object into an electric signal. The plane may be a plane of a window in the module housing or a plane near this window. This input device is characterized in that the converting means is constituted by the combination of the laser cavity and measuring means for measuring changes in operation of the laser cavity, these changes being due to interference of reflected measuring beam radiation re-entering the laser cavity and the optical wave in this cavity, and being representative of the relative movement of the object and the module.
By implementing the optical module in existing input devices, the input devices can be made simpler, cheaper and more compact. Moreover, new applications of input devices, especially in consumer products, become possible.
A first embodiment of the input device is characterized in that the measuring means is means for measuring a variation of the impedance of the laser cavity.
A preferred embodiment of the input device is characterized in that the measuring means is a radiation detector for measuring radiation emitted by the laser.
The radiation detector may be arranged in such a way that it receives part of the radiation of the measuring beam.
This embodiment of the input device is, however, preferably characterized in that the radiation detector is arranged at the side of the laser cavity opposite the side where the measuring beam is emitted.
Generally, diode lasers are provided with a monitor diode at their rear side. Usually, such a monitor diode is used to stabilize the intensity of the laser beam emitted at the front side of the diode laser. According to the invention, the monitor diode is used to detect changes in the laser cavity generated by radiation of the measuring beam re-entering the laser cavity.
An input device for measuring a movement of an object and the device relative to each other in a plane parallel to the illuminated surface of the object, is characterized in that it comprises at least two diode lasers and at least one detector for measuring the relative movement of the object and the device along a first and a second measuring axis, these axes being parallel to the illuminated surface of the object.
As will be explained later, this device and other devices utilizing two or more measuring beams, may be provided with a separate detector for each measuring beam. However, it is also possible to use one and the same detector for all measuring beams if time-sharing is used.
An input device which allows a third relative movement of the object and the device to be determined, is characterized in that it comprises three diode lasers and at least one detector for measuring a relative movement of the object and the device along a first, a second and a third measuring axis, the first and second axes being parallel to the illuminated surface of the object, and the third axis being substantially perpendicular to this surface.
This embodiment of the input device recognizes a single movement of the object and the device along the third measuring axis and converts it into an electric signal by means of which a click action may be determined.
An input device which allows determining both a scroll action and a click action is characterized in that it comprises two diode lasers and at lest one detector for measuring relative movements of the object and the device along a first measuring axis parallel to the object surface and along a second measuring axis substantially perpendicular to the object surface.
The first measuring axis is used to determine a scroll action and the second measuring axis is used to determine a click action.
Alternatively, this input device may be characterized in that it comprises two diode lasers and at least one detector for measuring relative movements of the object and the device along a first and a second measuring axis, these axes being at opposite angles with respect to a normal to the object surface.
The signals from both measuring axes comprise information about the scroll action and the click action, and by appropriately combining the information of the two measuring axes, the specific scroll action information can be isolated, as well as the specific click action information.
With respect to the constructional aspect, the input device may have several embodiments. A first embodiment is characterized in that the optical means comprises a lens arranged between said at least one laser and associated detector, on the one hand, and an action plane, on the other hand, the at least one laser being positioned eccentrically with respect to the lens.
An action plane is understood to mean a plane where a movement is measured, i.e., a plane where movement takes place and where measurement beams arrive. The action plane may be the plane of a window in the device housing or a plane near this window. The lens may be a rotationally symmetric lens or may have another shape. Due to the eccentric position of the lasers with respect to the lens element, it is ensured that corresponding illumination beams are incident on the window of the device at an acute angle so that these beams have a component along the associated measuring axis. For the following explanation, the term optical axis is introduced, which is understood to mean the symmetry axis of the lens, or the module, which axis is perpendicular to the window of the module.
If this embodiment comprises two diode lasers, it may be characterized in that the diode lasers are arranged such that the lines connecting their centers with the optical axis of the lens are at an angle of substantially 90xc2x0 with respect to each other.
If this embodiment comprises three diode lasers, it may be characterized in that the diode laser are arranged such that the lines connecting their centers with the optical axis of the lens are at angles of substantially 120xc2x0 with respect to each other.
The output signals of the detectors can be supplied to a common signal-processing circuit wherein at least two of the detector signals are used for each measuring axis to determine the movement along the relevant axis. In this way, more accurate values for the movements can be obtained.
In the input device, a diode laser of the type VCSEL (vertical cavity surface emitting laser) may be used. Such a laser emits radiation in the vertical direction, making it suitable for the present device. However, currently, since such a laser is quite costly, it is not very suitable for consumer mass products.
For this reason, preference is given to an input device which is characterized in that each diode laser is a horizontal emitting laser, and in that the device comprises, for each diode laser, a reflecting member reflecting the beam from the associated diode laser to an action plane.
Horizontal emitting diode lasers are the most commonly used lasers and are much cheaper than a VCSEL. Providing the device with a reflecting member adds little to the costs of this device.
An embodiment of the input device, which can be manufactured relatively easily and at low cost, is characterized in that it is composed of a base plate on which the at least one diode laser and associated detector are mounted, a cap member fixed to the base plate and comprising the window, and a lens accommodated in the cap member.
This embodiment is composed of only three portions which can be assembled easily and without severe alignment requirements.
An embodiment of the input device, which is even easier to manufacture, is characterized in that the lens is integrated in the cap member having an internal surface which is curved towards the base plate.
This embodiment is composed of only two portions.
These embodiments are preferably further characterized in that the base plate, the cap member and the lens are made of a plastic material.
Components made of such a material may be cheap and low weight and thus are suitable for consumer products. Only the material of the lens should be transparent and have some optical quality.
An alternative embodiment, i.e., without a lens, is characterized in that each diode laser is coupled to the entrance side of a separate light guide, the exit side of which is positioned at the window of the device.
In this embodiment, the radiation of an illumination beam is well isolated from its surroundings so that cross talk between the movements along different axes is eliminated or strongly is reduced.
This embodiment is preferably characterized in that the light guides are optical fibers. Optical fibers are flexible, have a small cross-section and show little attenuation per length unit, and thus allow location of the window of the device at a larger distance from the diode lasers and the detectors.
The embodiment with optical fibers is preferably characterized in that it comprises three diode lasers and three light guides, and in that the exit sides of the light guides are arranged in a circle at a mutually angular spacing of substantially 120xc2x0.
The input device may be used in different applications, such as, in a mouse for a desktop computer, in a keyboard of a desktop or laptop computer, in a remote control unit for different apparatus, in a mobile phone, etc.